1. Field of the Invention
The invention relates to an air-fuel ratio control apparatus for an internal combustion engine, and more particularly, to an air-fuel ratio control apparatus and an air-fuel ratio control method for an internal combustion engine that perform air-fuel ratio feedback control on the basis of a state of exhaust gas.
2. Description of the Related Art
As disclosed in Japanese Patent Application Publication No. 2002-276419 (JP-A-2002-276419), there is known a system in which an ammonia sensor is disposed in an exhaust passage of an internal combustion engine. In this system, the ammonia sensor is disposed at a post-stage of a catalyst disposed in the exhaust passage. Further, together with the ammonia sensor, an oxygen sensor is disposed at the post-stage of the catalyst.
NOx are likely to be contained in exhaust gas of the internal combustion engine when the air-fuel ratio of exhaust gas is lean. Thus, when the air-fuel ratio of exhaust gas continues to be lean, NOx may flow out to the post-stage of the catalyst. On the other hand, under a situation where the air-fuel ratio of exhaust gas is rich, NH3 (ammonia) is likely to be produced through a reaction of nitrogen in exhaust gas with hydrogen. Thus, under the situation where the air-fuel ratio of exhaust gas is rich, NH3 may be discharged to the post-stage of the catalyst.
The ammonia sensor is sensitive to NOx as well as NH3. Thus, the ammonia sensor disposed at the post-stage of the catalyst outputs a value corresponding to the concentration of NH3 under a rich atmosphere, and on the other hand, outputs a value corresponding to the concentration of NOx under a lean atmosphere.
The aforementioned system determines, on the basis of the output of the oxygen sensor disposed downstream of the catalyst, whether the air-fuel ratio of exhaust gas is rich or lean. Then, when the ammonia sensor outputs a value larger than a criterial value under a situation where the air-fuel ratio of exhaust gas is rich, this system determines that a large amount of NH3 has been generated, and attempts to make the air-fuel ratio lean. Further, when the ammonia sensor outputs a value larger than the criterial value under a situation where the air-fuel ratio of exhaust gas is lean, this system determines that a large amount of NOx has been generated, and attempts to make the air-fuel ratio rich.
According to the aforementioned processing, the air-fuel ratio of the internal combustion engine can be controlled such that the amounts of NH3 and NOx flowing out to a region downstream of the catalyst become sufficiently small. Thus, this system can ensure that the internal combustion engine acquires good emission properties.
However, for the first time when the ammonia sensor outputs a value larger than the criterial value under a lean atmosphere, the aforementioned system determines that the air-fuel ratio is deviant to a lean side, and makes the air-fuel ratio rich. According to this control, a certain amount of NOx inevitably flows out to the region downstream of the catalyst. In this respect, the aforementioned system leaves room for further improvement from the standpoint of the suppression of the discharge amount of NOx.